National ranking · most polluted air

Worst Air Quality in the US (2025)

Ranked by the lowest share of Good air days among counties with at least 100 monitored days in 2025, with higher median AQI breaking ties, from EPA Air Quality System monitoring.

60.0%
Non-Good days · #1 Marion, Indiana
33
Counties ranked

Marion County, Indiana has the worst 2025 Good-day share among 33 ranked U.S. counties (40.0% Good)

According to the U.S. EPA Air Quality System, this ranking orders 33 counties with at least 100 monitored days in 2025 by the lowest share of days in the Good AQI band, using median AQI only to break ties. It is an annual comparison of exposure patterns, not a live advisory. For current conditions and alerts, use AirNow.

Data source EPA Air Quality System (AQS)

#1 non-Good share
Marion County, Indiana ranks #1 of 33 for non-Good days at 60.0% (cohort median Good 77.5%).
#1 median AQI
#1 median AQI: 54
Counties ranked
Counties ranked: 33
Cohort median Good
Cohort median Good: 77.5%

from the compiled EPA AQS release in this database.

According to the U.S. Environmental Protection Agency (EPA), 977 counties reported Air Quality Index data through the federal Air Quality System (AQS) for the 2025 reporting year. The ranking below draws on those official records; see the methodology for how median AQI is computed.

Counties below the 100-day monitoring floor are excluded from this ranking: with only a handful of readings, a single bad-air stretch can collapse a county's Good-day share. Across all 951 floor-qualifying counties, the cohort median Good-day share is 77.5%; the first-ranked county records 40.0%. The gap therefore reflects a sustained annual pattern rather than a thin sample.

Sensitive group advisory

These counties have elevated exposure risk. People with asthma, COPD, heart disease, children, pregnant women, and adults 65+ should consult airnow.gov for real-time conditions and follow EPA AQI alerts before outdoor activity.

Filter this ranking

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Filters keep the Good-day-share ranking contract. A state filter ranks that state's qualifying counties; pollutant filters keep rows whose annual day-type count is led by ozone, PM2.5, or PM10. Raising the monitored-days floor tightens the sample beyond the default 100-day Module-34 guard.

Compare up to three counties

Showing 33 matching Indiana counties . Back to the national top 50.

Where the national worst cohort concentrates

Among the unfiltered national top 50 (lowest Good-day share, ≥100 monitored days), California contributes 13 counties , followed by Texas (11). Use the state filter above to rank every qualifying county inside one jurisdiction instead of truncating the national list.

Top contributing states in the worst-50

What leads the bad-air days

On each county row, EPA reports how many monitored days were ozone-, PM2.5-, or PM10-classified. In the national worst-50 cohort, PM2.5-led days are the plurality (32 of 50). Counts: ozone 16 · PM2.5 32 · PM10 2 . Filter by driver to keep only counties whose annual day-type count is led by that pollutant.

Ozone-led vs particle-led counties

Ozone-led rows (16) often sit in long warm-season photochemical corridors; PM2.5-led rows (32) more often track wildfire or winter particle episodes. Neither label is a health diagnosis - it is only which pollutant-day count dominated the annual monitor record.

How far below the cohort median

Across all 951 floor-qualifying counties, the median Good-day share is 77.5%. The #1 county here (Marion, Indiana) records 40.0% Good days - 37.5 percentage points below that median. That gap is why a thin sample is excluded: short monitor series can invent false extremes.

Annual record vs live advisory

This page answers “which counties had the worst annual Good-day share in 2025?” It does not answer “is the air safe today?” For current conditions and alerts use AirNow. State hubs at /states roll the same annual contract up one geography level.

Compare any two counties

Need a side-by-side annual record (Good-day share change + median AQI endpoints)? Use the compare form above or the county compare tool - it searches the full EPA AQS county set, not just this ranking slice.

Also useful nearby

Largest share of days outside the Good band

Largest share of days outside the Good band

100% minus Good-day share, 2025 (higher is worse)

  1. 1 Marion, Indiana 60%
  2. 2 Lake, Indiana 58.4%
  3. 3 Clark, Indiana 41.9%
  4. 4 Vanderburgh, Indiana 40.5%
  5. 5 Vigo, Indiana 40.3%
  6. 6 Allen, Indiana 40%
  7. 7 Elkhart, Indiana 39%
  8. 8 Porter, Indiana 38.1%
  9. 9 LaPorte, Indiana 37%
  10. 10 Hamilton, Indiana 36%
  11. 11 Whitley, Indiana 35.8%
  12. 12 Delaware, Indiana 35.3%

Non-Good monitored days · EPA Air Quality System

Ranked counties

Reading the table columns

Good Days % is the ranking key. Median AQI breaks ties. Max AQI flags extreme days (values above 500 are beyond EPA’s standard scale). Day driver is the pollutant with the most classified days on that county’s annual row.

# County State Good Days % Median AQI Max AQI
1 Marion Indiana 40.0% 54 Moderate 170
2 Lake Indiana 41.6% 53 Moderate 219
3 Clark Indiana 58.1% 46 Good 133
4 Vanderburgh Indiana 59.5% 47 Good 100
5 Vigo Indiana 59.7% 45 Good 108
6 Allen Indiana 60.0% 44 Good 115
7 Elkhart Indiana 61.0% 43 Good 171
8 Porter Indiana 61.9% 44 Good 136
9 LaPorte Indiana 63.0% 43 Good 153
10 Hamilton Indiana 64.0% 44 Good 118
11 Whitley Indiana 64.2% 39 Good 108
12 Delaware Indiana 64.7% 43 Good 108
13 Floyd Indiana 65.8% 44 Good 126
14 St. Joseph Indiana 66.0% 43 Good 127
15 Howard Indiana 66.6% 43 Good 122
16 Spencer Indiana 67.4% 39 Good 88
17 Dubois Indiana 69.7% 39 Good 88
18 Henry Indiana 69.9% 37 Good 95
19 Tippecanoe Indiana 70.3% 39 Good 95
20 Madison Indiana 71.2% 41 Good 122
21 Greene Indiana 72.0% 42 Good 90
22 Bartholomew Indiana 72.6% 43 Good 97
23 Monroe Indiana 73.6% 36 Good 84
24 Warrick Indiana 87.4% 37 Good 97
25 Boone Indiana 89.7% 35 Good 112
26 Posey Indiana 90.5% 35 Good 105
27 Carroll Indiana 90.8% 35 Good 105
28 Knox Indiana 91.3% 36 Good 93
29 Shelby Indiana 91.4% 35 Good 105
30 Hendricks Indiana 92.0% 35 Good 90
31 Wabash Indiana 92.0% 34 Good 112
32 Perry Indiana 92.7% 36 Good 84
33 Brown Indiana 95.9% 34 Good 77

Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS). Median AQI is calculated from daily AQI readings across all available monitoring days. Unhealthy Days includes Unhealthy, Very Unhealthy, and Hazardous categories

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Every figure on PlainAirData is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.

Informational only, not professional advice. Data from EPA AQS. See our disclaimer.